A person or device that attaches, adds, or appends something to another thing.
From append (to attach or add) + -er (one who does something). The verb append entered English via Latin appendere, and the agent noun -er was added to create the person or thing that performs the action.
Notice how English loves the -er suffix—baker, teacher, appender. It's so productive that you can theoretically make an 'er' noun from almost any verb, even if it's not in the dictionary. Language is democratic that way!
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